Method for inhibiting corrosion



April 1953 e. H. ROHRBACK ETAL 2,635,698

METHOD FOR INHIBITING CORROSION Filed March 16, 1951 2 SHEETS-SHEET l TUBING FILLED WITH FlC;.l-BATCH TREAT NT IN RWC-l 70 I 60 INJECTION STARTEDIILB.SOD.

/ARSENITE DAILY (11 PPM AS203) F) a: 50 BEFORE TREATMENT 11 AVERAGE 9 SAMPLES E 40 V m l E 30 2L5. s00. ARSENITE DAILY D (35 PPM A5203) 8 20 z o I l0 o o s m I5 20 25 DAYS OF INHIBITOR TREATMENT FIGZ ARSENITE INJ ECTION IN D-S-NO.2

INVENTORS G/LSON ROHRBACK DW/TE M. MCCLOUD April 21, 1953 G. H. ROHRBACK ETAL METHOD FOR INHIBITING CORROSION Filed March 16, 1951 IRON COUNT-PPM F8203 2 SHEETSSHEET 2 INJECTION STARTED12LB. s00. ARSENITE DAILY (20 PPM AS203) BEFORE TREATMENT AVERAGE 9 SAMPLES o o o o 5 IO 15 2o 25 DAYS OF INHIBITOR TREATMENT F|G.3-ARSENITE INJECTION IN BC-|7 D BEFORE TREATMENT AVERAGE IO SAMPLES INJECTION STARTEDIZLB. s00. ARSENITE DAILY (lo PPM A5203) o 5 l0 I5 20 25 DAYS OF INHIBITOR TREATMENT FlG.4-ARSEN|TE INJECTION IN B-28 INVENTORS ATTO EYS

Patented Apr. 21, 1953 UNITED STTS ATENT OFFICE METHOD FOR INHIBITING CORROSION Application March 16, 1951, Serial No. 216,079

Claims. 1

This invention relates to a method for inhibiting the corrosion of ferrous metals constituting the flow lines of producing oil wells. More particularly, the invention relates to a method for preventing corrosion of ferrous metals in a producing oil well delivering a production stream comprising crude oil, brine consisting mainly of aqueous sodium chloride and commonly containing small amounts of other inorganic salts, and carbon dioxide.

Corrosion of ferrous metal surfaces in contact with the production streams of producing oil wells has long been recognized as a serious operating problem by petroleum producers. A considerable research effort has been directed to this problem, and several methods of reducing corrosion rates and thereby reducing the frequency of replacement of well tubing, sucker rods, etc. have been proposed. In the present state of the art it appears that the attack upon any given corrosion problem must of necessity be in a very considerable degree empirical. To illustrate, corrosion inhibitors which have been found effective in the acid media of pickling baths cannot be expected to exhibit an equivalent inhibiting efiect in a different corrosive environment. Many of the inhibitors which are effective in pickling baths are found to accelerate corrosion under the conditions which exist in a producing oil well. Further, a corrosion inhibitor which is effective in a particular well or field is frequently found to be entirely ineffective and even to accelerate corrosion in a different well or field.

The particular corrosive environment with which the present invention is concerned is one in which corrosion by direct acid attack upon the ferrous metal is a minor factor in producing the serious total corrosion with which the producer is confronted. The essential features of this corrosive environment include a ferrous metal in contact with oil, brine, and gas containing carbon dioxide, gas liquid interfaces in contact with the metal, and areas of turbulent liquid flow in contact with the metal surfaces.

The metal in this environment is corroded away by direct attack of carbonic acid which is greatly accelerated by electrochemical phenomena arising out of the contact of the phase interfaces with the metal and out of the contact of turbulent flowing liquid with the metal surface. Substantially all of the acidity of the production stream stems from the presence of carbon dioxide. The acidity is quite low, the pH not being lower than 3 and being ordinarily in the range 4 to 6. When a ferrous metal is exposed to the production efiiuent under quiescent conditions where there is no movement of the liquid or of free gas bubbles through the liquid, the corrosion rate observed is usually less than about 25% of that observed under actual producing conditions which include turbulent motion of the liquid in contact with the metal and the presence of minute gas bubbles which contact the metal. In laboratory apparatus in which the separate influence of the three corrosive factors was studied, i. e., direct acid attack, electrochemical attack attributable to turbulent liquid flow, and electrochemical attack attributable to the presence of finely divided gas bubbles which contact the metal surfaces, it was found that the relative rate of corrosion due to direct acid attack was l units, and that the relative rate of corrosion when either the condition of liquid turbulence or finely divided gas bubbles moving through the liquid was superimposed on the presence of the acid medium, the relative corrosion rate immediately increased to 14 units. From these observations it is clear that the corrosion problem would not be solved by the employment of an inhibitor which would eliminate only the corrosive action due to direct acid attack and that the corrosion problem here presented can be solved only if the additional electrochemical corrosion attributable largely to physical conditions is markedly reduced. The fact is, that if the only corrosion encountered in this corrosive environment were the corrosion atributable to direct acid attack, there would be no corrosion problem and corrosion at the low rate produced by this source alone could be accepted since other operating factors would then be controlling in fixing equipment life and breakdown frequency.

Considerable effort has heretofore been directed to the prevention of corrosion in the corrosive environment with which the present invention is concerned. The most practical solution to this problem has heretofore been developed, i. e., it has been found that if a concentration of about p. p. m. of sodium dichromate based on produced water is maintained in the well, the rate of corrosion is appreciably reduced. However, the employment of sodium dichromate gives rise to a new and serious problem, especiall where the treated well is delivering a crude oil having an A. P. I. gravity below 18 and where the production stream consists of this oil, brine, and a gas containing carbon dioxide. The employment of sodium dichromate in this type of well gives rise to a serious emulsion problem. It is commonly found that a well of this type which normally produces practically no water-oil emulsion will, upon the addition of 50 p. p m. of sodium dichromate, deliver a production stream in which upwardly of 50% of the oil, and frequently '75 to 80% of the oil, is in the form of a stable waterin-oil emulsion. In this situation the producer is confronted with a difficult choice between two evils.

It is an object of this invention to provide a method for inhibiting corrosion of ferrous metal tubing and the like in producing oil wells, de livering a production stream comprising crude oil, brine, and carbon dioxide gas.

It is a further object of this invention to provide a method for inhibiting corrosion of ferrous metal tubing and the like in oil wells delivering a production stream comprising crude oil, brine, and carbon dioxide gas without the concurrent production of stable oiL-in-water emulsions.

It has now been found that corrosion of ferrous metal surfaces in a producing oil well delivering a production stream comprising crude oil, brine, and carbon dioxide can be very markedly reduced by introducing a water solution of an inorganic arsenous compound into the well bottom. The arsenous compound may be either continuously or intermittently lubricated into a Well having an open annulus between the liner and tubing and must be intermittently introduced through the tubing of a well in which this annulus is sealed off. The amount of the arsenic compound introduced is determined by the production of water in a particular well and is an amount sufficient to give the produced water an average arsenic content in the range to 50 p. p. m. calculated as arsenous oxide. When the arsenou compound is intermittently introduced into the well, the quantity introduced in any single specific instance is sufficient to give the total water produced by the well from the time of that specific instance until the next instance of introduction an average arsenic content in the range 10 to 50 p. p. m. calculated as arsenous oxide. In a desirable embodiment of the invention arsenic concentrations in the range 10 to 50 p. p. m. are maintained as above described fora period of 100 to 600 hours, during "which time the corrosion rate is markedly reduced and plateaus at a low level. After this period the rate of arsenic introduction iscut back to 1 to p. p. m. as AS203 which is sufficient to maintain the corrosion rate at the said low level.

The water-soluble inorganic arsenous compounds which may be employed in the process of this invention include the arsenous halides and arsenous oxide. Arsenous oxide'is rather insoluble at ordinary temperatures, but may be introduced into the well in the form of a slurry which goes into solution quite readily at the higher temperatures "prevailing at the'well bottom. The preferred arsenous compounds, however, are solutions of arsenous oxide in aqueous alkaline materials. Arsenous oxide may be dissolved in aqueous 'solutions of the alkali metal hydroxides, the alkaline earth metal hydroxides, and salts of the alkali metals and alkaline earth metals with weak inorganic acids. For example, aqueoussodium hydroxide, aqueous potassium hydroxide, aqueous magnesium hydroxide, aqueous calcium hydroxide, aqueous sodium carbonate, and the like, may be employed to dissolvearsenous oxide to constitute a suitable'inhibitor solution. Where diflicultly soluble hydroxides or salts are employed, the arsenic oxide may be stirred into.

slurries of these materials in Water to produce a concentrated solution of arsenous oxide in the slurry.

The amount of arsenous oxide dissolved in the aqueous alkaline solutions of the character above described may be varied over a considerable range from amounts less than the amount of arsenous oxide stoichiometrically equivalent to the alkaline material in solution, to amounts of arsenous oxide substantially greater than the stoichiometric amount required for formation of a corresponding arsenite with the alkaline material. The resulting solutions in all cases contain alkali metal arsenites and alkaline earth metal arsenites and complex arsenites. However, the inhibitor solu tions may be best generically described as solutions of arsenous oxide in aqueous alkali media.

An aqueous solution of arsenous oxide in sodium hydroxide containing about 4 pounds of arsenous oxide per gallon of solution and about 8% by Weight of titratable sodium hydroxide is a suitable inhibitor concentrate for storage at the wellhead and introduction into the well together with additional water. A very useful concentrate may be prepared by dissolving 4 pounds of arsenous oxide and 1 pound of sodium hydroxide in 7.2 pounds of water to form one gallon of inhibitor solution.

This concentrate may be either pumped or lubricated into an oil Well annulus or tubing for corrosion mitigation. The amount of inhibitor solution to be used for effective treatment in a particular well depends on the rate of water .production of that well. Although slight differences may be found among oil Wells, the injection procedure recommended below will be satisfactory for most wells.

Injection quantities of concentrate If an injection pump is'used to feed the inhibitor into the Well-annulus, the feed stream should be kept continuous. The inhibitor should be di- Iuted with Water in a 'feed tank and then pumped in the well. At least 30=gallons of watershouldbe used in the daily injection solution and up to gallons of water daily maybe used. This insures a good washing of the inhibitor down the .annulus.

If a liquid lubricator is used to feed the inhibitor into the well annulus, injection should be made daily for the first six weeks. Thereafter, corrosion protection may be maintained inmost wells by less frequentinjection. Large quantities of water should ,be used to wash the inhibitor down the annulus. The amount of water .used may be limitedby the :type' of lubricator, but at least 30 gallonss'hould-beused.

Another treatment method -that may be used for the concentrate "is periodic "batch lubrication of water solution directly into the tubing. "This method is particularly useful for treatment of flowing wells with ;packed -oif -annulus. A sug gested tre'atment isas follows:

(1) Dilute the concentrate to form a solution having 0;01-:% to .-2.0.% by weight content-of dis solved AS203. Pump the dilute solution into the tubing, while the well is shut in, in amount sumcient to fill the greater part of the tubing.

(2) Allow solution to stand in tubing at least one hour.

(3) Force the solution into the formation and then resume production.

(4) Repeat treatment when iron count (a determinant of corrosion rate hereinafter described) rises to 50 per cent of uninhibited average. After about six weeks of treating, the frequency of treatment is reduced to about one treatment per month.

The batch treatment above described was field tested in a corrosive well delivering a production stream comprising salt water, oil, and gas containing carbon dioxide. The well was 9000 feet deep and the inside diameter of the tubing was 2 inches. The corrosion inhibitor concentrate used in the batch test contained four pounds per gallon of arsenous oxide and one pound per gallon of sodium hydroxide. 25 gallons of this solution and 60 barrels of salt water were mixed and forced into the well tubing using a high pressure pump. This concentrated solution containing approximately 4,000 parts per million of arsenous oxide was held in the tubing for a period of about 2 hours. The tubing stream was thus coated completely with a concentrated inhibitor solution. The inhibitor solution was then displaced into the formation by pumping oil down the tubing. After sufdcient oil had been forced down the tubing to completely displace the inhibitor solution into the formation, production was resumed. The effectiveness of the inhibitor treatment was determined by ascertaining the iron content of the produced water before and after treatment. The iron content was expressed in terms of iron count, defined below. The water resident in the formation in the wells tested by the batch method contained iron in solution as determined by analysis of the bottom hole samples. During the flow of the production stream through the tubing from the formation to settling tanks, the iron content of the water was markedly increased by corrosion of the tubing and piping. Before the commencement of the batch treatment, the pickup in iron count during the passage of the water contained in the production stream through the tubing and piping of the well amounted to 15 iron count units. After the batch treatment the iron count pickup of the produced water was determined daily for a period of 8 days, during which the average pickup was 1 iron count unit. This indicated that 93% of the corrosion of the tubing was eliminated by the batch inhibitor treatment and that this reduced corrosion persisted over a period in excess of 13 days. One month later the iron count pickup was still substantially below the original value. Th total inhibitor solution used in this batch treatment was sufiicient to give the total volume of water produced in 30 days an arsenic content of about 25 p. p. m. calculated at AS203. The persistence of the corrosion reduction which attends this method of treating is very attractive to the field producer, since it means that production need be interrupted only at infrequent intervals to introduce the inhibiting solution and, further, that a small service crew can service a large number of wells in a field since a single well need be attended only about once a month. The data from the batch treatment is graphically represented in Figure 1 of the appended drawings.

In the drawings, Fig. 1 is a graphical representation of a batch treatment or a-particular well with arsenous oxide. Figs. 2 to 4 inclusive graphically represent the results of field tests in other wells in which solution of arsenous oxide in sodium hydroxide were injected into the wells, the amounts of solution injected containing from 10 to 35 parts of arsenous oxide to each million parts of water produced by the wells.

Field tests were conducted in three wells delivering a production stream comprising crude oil, brine, and carbon dioxide gas. The results of these field tests are graphically presented in Figures 2 to 4, where the iron count of the water produced by the well is plotted against the time of treatment at the rates shown. The iron count, a measure of the corrosion rate, is the number of parts per million of ferric oxide contained in the produced water as determined by the thiocyanate colorometric method (Scott's Standard Method of Chemical Analysis, 5th ed., Van Nostrand, 1939, page 486') From the data presented in the graphs it is evident that treatment pursuant to the invention eliminates approximately 90% of the corrosion normally sustained in the corrosive environment heretofore specifically described.

In the field tests summarized in Figures 1 to 4 mentioned above, no emulsion formation resulted from the introduction of the inhibitor into the well.

For the purpose of comparing the emulsionforming propensities of the inhibitor of the present invention and sodium dichromate, a well normally producing a stream comprising crude oil having an A. P. I. gravity of 17, brine, and about 10,000 cu. ft. per day of gas containing about 3% carbon dioxide and normally free of emulsion difficulties was selected for comparative tests. In a first test sodium dichromate in amount equivalent to 50 p. p. m. based on produced water was introduced into the well. Immediately following its introduction, a water-in-oil emulsion appeared in the production stream and when a steady state developed in the production stream it was found that 70 to of the total oil produced was in the form of a water-in-oil emulsion which did not break on standing.

The sodium dichromate introduction was discontinued for ten days, during which the corrosion rate rose to its normal level and emulsion difficulty disappeared. Sodium arsenite was then introduced in the manner described above in an amount sufficient to give produced water an arsenic content of 20 p. p. m. calculated as arsenous oxide. The corrosion rate rapidly decreased by an average of about 92% and no emulsification developed.

Thi application is a continuation-in-part of our earlier application, Serial No. 203,264, filed December 29, 1950.

Obviously, many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof, and only such limitations should be iniposed as are indicated in the appended claims.

We claim:

1. The method of inhibiting the corrosion of ferrous metal tubing in a producing oil well delivering a production stream comprising crude oil, brine, and carbon dioxide gas and characterized by a pH above 3 through said tubing, which comprises introducing a solution of arsenous oxide in aqueous alkali into the well bettom, the amount of the arsenous compound introduced being equivalent to that required to occasion being equivalent to that required to give the Water produced subsequent to the particular occasion and prior to the next succeeding occasion of introduction an arsenic concentration in the range of about 1 'to 50' p. p. m. calculated as arsenous oxide.

3.. The method of inhibiting acid and electrochemical corrosion of ferrous metal tubing in a producing oil well delivering a production stream comprising crude oil, brine, and carbon dioxide gas and characterized by a pH above 3 through said tubing, which comprises introducing a solution: of arsenous. oxide in aqueous sodium hydroxide into the well bottom, the amount of the solution introduced being sufiioient to give the produced water an arsenic-concentration in the range about 120 to 50 p. 'p. m. calculated as arsenous oxide.

4. The. method of inhibiting the corrosion of ferrous metal'tubing in. a producing oil well delivering a production stream comprising crude oil, brine, and carbon dioxide gas and characterized by .a pH above 3 through said tubing, which comprises periodically halting the production of the well, filling the tubing with a solution. of are senous oxide in aqueous alkali, the solution having an arsenous oxide content in the range 0.01% to 2.0% by weight, maintaining the solution in residence in the tubing for at least one hour, forcing the solution from the tubing into the formation, and producin the well.

'5. The method of inhibiting the corrosion of ferrous metal tubing in a producing oil well delivering a production stream comprising crude oil, brine, and carbon dioxide gas and characterized by a pH above 3 through said tubing, which comprises introducing a solution of arsenous oxide in aqueous alkali into the well bottom and producing the well, the amount of the solution introduced during the first 100 to 600 hours of treatment being equivalent to that required to give the produced water an arsenic content in the range 10 to p. p. m. calculated as arsenous oxide and the amount introduced thereafter being a lesser amount equivalent to that required to give the produced water an arsenic content in the range 1 to 15 p. p. m. calculated as arsenous oxide.

GILSON H. RO-H-RBACK. DWITE M. MCCLOUD.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,877,504 Grebe Sept. 13, 1932 1,938,961 Gravell Dec. 12, 1933 2,207,733 Hei'iey July 16, 1940 

5. THE METHOD OF INHIBITING THE CORROSION OF FERROUS METAL TUBING IN A PRODUCING OIL WELL DELIVERING A PRODUCTION STREAM COMPRISING CRUDE OIL, BRINE, AND CARBON DIOXIDE GAS AND CHARACTERIZED BY A PH ABOVE 3 THROUGH SAID TUBING WHICH COMPRISES INTRODUCING A SOLUTION OF ARSENOUS OXIDE IN AQUEOUS ALKALI INTO THE WELL BOTTOM AND PRODUCING THE WELL, THE AMOUNT OF THE SOLUTION INTRODUCED DURING THE FIRST 100 TO 600 HOURS OF TREATMENT BEING EQUIVALENT TO THAT REQUIRED TO GIVE THE PRODUCED WATER AN ARSENIC CONTENT IN THE RANGE 10 TO 50 P.P.M. CALCULATED AS ARSENOUS OXIDE AND THE AMOUNT INTRODUCED THEREAFTER BEING A LESSER AMOUNT EQUIVALENT TO 